Apparatus for processing fluid materials particularly in the preparation of samples for radioactive isotope tracer studies

ABSTRACT

Method and apparatus for the processing of fluid materials, particularly in the preparation of samples for radioactive isotope tracer studies by combustion of starting materials containing such isotope tracers. The sample is burned in a combustion chamber which tapers upwardly and inwardly above the sample receptacle so as to approximate the shape of the flame of a burning sample, and the combustion products are continuously exhausted from the combustion chamber and passed through a heat exchanger which condenses the condensable vapors in the combustion products. The condensed vapors are then separated from the gases, and the gases are passed into a reaction column if there is a radioactive isotope tracer remaining in gas form. Oxygen is fed into the combustion chamber at a controlled rate during combustion, and after combustion nitrogen gas is fed into the combustion chamber and exhausted therefrom through the heat exchanger and into the separating means, so as to purge the system of any remaining gaseous production products. A liquid scintillator, and a liquid solvent if desired, are passed through the heat exchanger into the separating means after each combustion so as to recover any residual condensed vapors. In the reaction column, the gas containing the radioactive isotope tracer is reacted with a trapping agent in a column comprising a series of smoothly contoured reaction chambers interconnected by smoothly contoured necked down portions. After all the gases have been passed through the column, the direction of gas flow is reversed in the column so as to discharge the reaction product into a counting vial. A liquid scintillator, and a liquid solvent if desired, are then passed through the reaction column following the same procedure as followed previously for the trapping agent, thereby recovering all the reaction product from the column.

This is a division of application Ser. No. 706,499, filed July 19, 1976,now U.S. Pat. No. 4,148,608, which in turn is a divisional ofapplication Ser. No. 277,261, filed Aug. 2, 1972, now U.S. Pat. No.3,979,503, which in turn is a continuation of application Ser. No.728,939, filed May 14, 1968, now abandoned.

The present invention relates generally to the processing of fluidmaterials. In its principal application, the invention relates tomethods and apparatus for the preparation of samples for radioactiveisotope tracer studies and, more particularly, to an improved method andapparatus for preparing such samples by combustion of the startingmaterial containing the isotope tracer.

It is a primary object of the present invention to provide an improvedmethod and apparatus for the preparation of samples for radioactiveisotope tracer studies, which reduce the sample preparation time farbelow the preparation times required by the methods and apparatuspreviously known for the preparation of such samples, with correspondingincreases in the sample preparation rate. In this connection, a relatedobject of the invention is to provide such an improved method andapparatus which permit a technician to prepare a much greater number ofsamples in any given work period, thereby improving the efficiency andreducing the cost of such preparation procedures.

Another important object of the present invention is to provide animproved sample preparation method and apparatus of the foregoing typewhich significantly increase the efficiency of the isotope recovery fromthe starting material. More particularly, it is a specific object ofthis invention to provide such a method and apparatus which are capableof recovering essentially 100% of the isotope present in the startingmaterial.

A further significant object of this invention is to provide an improvedsample preparation method and apparatus of the type described abovewhich virtually eliminate the "memory" of the apparatus so that theamount of isotope tracer in any given sample prepared thereby issubstantially independent of any previous preparations carried out inthe same apparatus. Thus, a related object of the invention is toprovide such an improved method and apparatus which greatly improve thereliability of the resultant samples and the data derived therefrom.

Still another object of the invention is to provide such an improvedsample preparation method and apparatus which are capable of providingradioactive samples containing little or no oxygen, thereby minimizingthe quenching effects caused by oxygen in such samples during analysisthereof. Accordingly, a related object of the invention is to providesuch an improved method and apparatus which facilitate analysis of theresulting samples.

A specific object of one particular aspect of the invention is toprovide an improved combustion chamber for burning liquid or solidsamples in an open system so that the combustion products arecontinuously removed from the combustion chamber, and including meansfor facilitating thorough cleaning of the chamber in a rapid andefficient manner after each combustion. Thus, a related object of thisaspect of the invention is to provide such an improved combustionchamber which has virtually no memory, even when used to burnradioactive samples.

In another aspect of the invention, it is an object to provide animproved heat exchanger for condensable vapors which provides anextremely high heat transfer with only a small volume and surface areaand in a very short time period. In this connection, it is also anobject of this aspect of the invention to provide such an improved heatexchanger which is capable of receiving the products of combustion of aradioactive sample and continuously condensing the vapors therefrom in arapid and efficient manner.

Yet another object of a further particular aspect of the invention is toprovide an improved gas-liquid reaction column which achieves a highreaction rate between the gas and liquid, and yet can be thoroughlycleaned in a matter of seconds between successive batch-type reactionsso that the column has virtually no memory, even when used to reactradioactive materials.

Another important object of the present invention is to provide animproved radioactive sample preparation apparatus which achieves all theobjectives mentioned above at a low cost and with the use of highlyreliable components, and which is easy to operate.

Other objects and advantages of the invention will become apparent fromthe following detailed description and upon reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a sample preparation system embodyingthe present invention, for use in the preparation of samples forradioactive isotope tracer studies;

FIG. 2 is an elevation view, partially in section, of combustionapparatus for use in the sample preparation system of FIG. 1, andincluding a schematic diagram of a portion of the fluid and electricalsystems associated therewith;

FIG. 3 is a sectional elevation view of a heat exchanger and condensateseparation means for use in the sample preparation system of FIG. 1, andincluding a schematic diagram of a portion of the fluid systemassociated therewith; and

FIG. 4 is a sectional elevation view of a reaction column for use in thesample preparation system of FIG. 1 and including a schematic diagram ofa portion of the fluid system associated therewith.

While the invention will be described in connection with certainpreferred embodiments, it will be understood that it is not intended tolimit the invention to these particular embodiments. On the contrary, itis intended to cover all alternatives, modifications and equivalentarrangements as may be included within the spirit and scope of theinvention as defined by the appended claims.

Turning now to the drawings, in FIG. 1 there is illustrated a samplepreparation system for use in the preparation of samples for radioactiveisotope tracer studies, such as studies involving tissue distributionand residue levels of drugs in plants and animals. In the preparation ofsuch samples, a sample of the starting material containing theradioactive isotope tracer, such as a sample of the plant or animaltissue, is burned to convert the carbon in the starting material tocarbon dioxide and the hydrogen to water, and the radioactive isotopetracer is then recovered from the resulting combustion products. Forexample, if the particular radioactive isotope tracer employed is ¹⁴ C,it appears in the combustion products as ¹⁴ CO₂ gas; if the tracer istritium (³ H), it appears in the combustion products as ³ H₂ O in theform of a condensable vapor. Although ¹⁴ C and ³ H are the most commonlyemployed tracers, it will be understood that a number of otherradioactive isotopes may be employed, such as ³⁵ S which is converted tosulfate during combustion.

In order to provide samples which can be analyzed for radioactivity, thecompounds containing the isotope tracers are recovered from thecombustion products, and separated from any materials therein whichmight interfere with the radioactivity determination. For example, the ³H₂ O is recovered by cooling the combustion products to condense thevapors therein, including the ³ H₂ O, after which the condensed vaporsare separated from the remaining gases. The ¹⁴ CO₂ may also be recoveredby condensation or freezing at extremely low temperatures, such as bythe use of liquid nitrogen for example, but it is more conventional toreact the ¹⁴ CO₂ with a liquid trapping agent such as ethanolamine; theresulting reaction product is then recovered and mixed with a liquidscintillator to provide a sample suitable for use in making aradioactivity determination.

Referring now more specifically to FIG. 1, the sample to be burned isplaced in a sample basket 10 which forms a part of the electricalignition system, and also functions as a catalyst for efficientcombustion for the sample contained therein. The basket 10 is suitablymade of platinum or a platinum-rhodium alloy, so that the basket can beused both as an electrical resistor in the ignition system and as acatalyst for the combustion of the sample. A pair of electricalconductors 11 and 12 extend upwardly from a mounting plate 13, tosupport the basket 10 at the upper and lower ends thereof, while alsomaking electrical contact with the basket to connect it into theelectrical ignition system. The conductors 11 and 12 extend verticallydown through the plate 13 and terminate in depending connector pinsbeneath the plate 13.

In order to facilitate the loading of successive samples, the mountingplate 13 is supported on the top of a small platform 14 threaded on tothe end of a pneumatic piston rod 15. To load a sample in the basket 10,the pneumatic cylinder and piston assembly 16 associated with the rod 15is actuated to retract the piston rod 15, thereby lowering the basket 10through an opening 17 in the bottom of a combustion chamber 18. Thesample is then loaded in the basket, and the cylinder and pistonassembly 16 is actuated to advance the rod 15 and thereby raise thebasket 10 through the opening 17 into the combustion chamber 18. As theplatform 14 enters the opening 17, a sealing ring 19 mounted in a groovein the outer periphery of the platform 14 engages the tapered walls ofthe opening 17 to form a gas-tight seal therewith, as shown in FIGS. 1and 2.

For the purpose of igniting a sample contained in the basket 10 after ithas been raised into the combustion chamber 18, the connector pinsdepending from the plate 13 fit into complementary electricalreceptacles 20 in the top of the platform 14. The receptacles 20, inturn, are connected to an electrical igniter circuit including a powersource such as battery 21 and an ignition switch 22 for applying anelectrical voltage across the basket 10, which serves as a resistivetype heating element in the igniter system. Thus, the sample is ignitedby simply closing the switch 22, which is opened again as soon ascombustion has been initiated.

In order to supply the oxygen required for combustion of the samplecontained in the basket 10, pure oxygen is supplied to the combustionchamber 18 through a valve 23, a flow meter 24, and a pair ofcooperating passageways 25 and 26 formed in the platform 14 and theplate 13. The gas discharge passageway 26 in the plate 13 is positioneddirectly beneath the center of the basket 10, so that the oxygen is feddirectly into the combustion zone. The oxygen flow rate is adjusted, viathe valve 23 and flow meter 24, to a level slightly above that requiredto support combustion of the sample in the basket 10, so that there is aslight excess of oxygen within the combustion chamber. Consequently,there is generally a relatively thin layer of an oxygen-rich atmospherebetween the combustion flame and the inside walls of the combustionchamber 18, as indicated by the arrows in FIG. 1. This excess oxygenrises through the combustion chamber and is exhausted from thecombustion chamber 18 along with the combustion products through alateral exit 27 at the top of the chamber.

In accordance with one aspect of the present invention, the combustionchamber is open at the upper end thereof with the sidewalls extendingupwardly and inwardly above the sample basket so as to approximate theshape of the flame of a burning sample, thereby minimizing the volume ofoxygen-rich atmosphere around the flame, and the walls of the combustionchamber are preheated so as to maintain the wall temperature above thecondensation temperature of the vapors contained in the combustionproducts. With this design, the combustion products tend to be sweptdirectly into the exit 27, with the rising layer of oxygen-richatmosphere along the chamber sidewalls tending to isolate the combustionproducts from the sidewalls. Moreover, any combustion products that docontact the chamber walls remain in the gas state, even duringinitiation of the combustion, because the walls are pre-heated andmaintained at a temperature above the condensation temperature. Thus, inthe illustrative embodiment of the combustion chamber illustrated inFIGS. 1 and 2, the walls of the combustion chamber 18 extend verticallyupwardly past the sample basket 10, and then slope inwardly above thebasket so as to approximate the shape of the flame represented in brokenlines. Surrounding the combustion chamber 18 is a cylindrical vessel 30which defines an annular cavity around the outer surface of the chamber18 for receiving a preheating fluid. To center the combustion chamber 18within the vessel 30, the upper end thereof meshes with a complementarymounting element 31, while the lower end fits into a complementary holein the bottom wall of the vessel 30.

Prior to ignition of the sample contained in the basket 10, the fluidcontained in the annular cavity between the combustion chamber 18 andthe vessel 30 is heated by means of a heating coil 32 at the lower endof the cavity. The fluid distributes this heat along the walls of thecombustion chamber 18 so that the walls are uniformly heated to atemperature above the condensation temperature of the vapors containedin the combustion products to be produced. It has been found that thepreheating of the combustion chamber walls to maintain the combustionproducts in gaseous form even during ignition, combined with theflame-shaped configuration of the chamber, permits the combustionproducts to be exhausted from the combustion chamber, on a continuousbasis, so efficiently that there is virtually no residue of combustionproducts deposited on the chamber walls. The illustrative system alsoprevents condensation within the exit 27 of the combustion chamber 18,since the exit is also surrounded by the preheated fluid in the annularcavity between the combustion chamber 18 and the surrounding vessel 31.

As the exhausted gases leave the exit 27, they enter a transfer tube 34which is insulated to maintain the fluids passing therethrough in agaseous state. In the particular embodiment illustrated, the transfertube 34 is double walled with a metallic inner shell and an insulatingouter shell to minimize the heat loss therethrough. From the transfertube 34, the gaseous combustion products are passed through a Tconnection 40 into a heat exchanger 41 for cooling the exhaustedcombustion products to condense the vapors therein. The heat exchanger41 includes an inner member 42 forming a fluid passageway for receivingthe combustion products from the tube 34, and an outer shell 43 definingan annular cavity around the inner member 42 for receiving a coolingliquid to maintain the walls of the inner passageway at a temperature atleast as low as the condensation temperature of the vapors passingtherethrough. When the radioactive isotope tracer is in the form of acondensable vapor, such as ³ H₂ O for example, the heat exchanger 41functions to convert the tracer from a vapor to liquid form. In caseswhere the radioactive isotope tracer is in the form of a gas to bereacted with a trapping agent, for example, the heat exchanger 41functions to remove the condensable vapors from the tracer gas before itis reacted with the trapping agent.

In accordance with another significant aspect of this invention, thefluid passageway of the heat exchanger is, formed of thermallyconductive material designed to provide laminar flow of gases and vaporspassing therethrough in the absence of condensation, and the crosssection of the fluid passageway is sufficiently small in at least onedirection transverse to the fluid flow to provide capillary attractionon the type of liquid condensed within the passageway. Thus, in onepreferred embodiment of the invention, the inner member 42 comprises astraight thin walled metal tube having an inside diameter of about 0.05inch, with a wall thickness of about 0.004 inch, and a length of about 5inches. Although both the volume and the heat transfer surface area ofsuch a tube are obviously very small, it has been found that such a heatexchanger is capable of reducing the temperature of the combustion gasesto the condensation temperature with such a high degree of efficiencythat virtually 100% of the condensable vapors can be recovered in liquidform at the outlet end of the heat exchanger. Moreover, this heattransfer is effected without producing a high backpressure or otherwiseinhibiting the exhaustion of the combustion products from the combustionchamber directly upstream of the heat exchanger inlet.

Although it is not intended to limit this aspect of the invention to anyparticular theory, it is believed that the fluid passageway designed inaccordance with this invention causes droplets of liquid condensate toform along the walls of the passageway, thereby providing extremelyefficient heat transfer conditions. This drop-wise condensation may becaused or promoted by the capillary nature of the fluid passageway. Whenthe fluid passageway in the heat exchanger is in tubular form as in theillustrative embodiment, a pulsating pressure is detected at the inletof the passageway, and it is believed that drop-wise condensation mayaccount for this pulsating pressure. It will be appreciated, however,that the fluid passageway may have forms other than tubular, such as anarrow slot, since capillary attraction is present whenever the surfaceof a liquid where it is in contact with a solid is elevated by therelative attraction of the molecules of the liquid for each other andfor those of the solid.

As another feature of the present invention, a separating means isconnected to the outlet end of the heat exchanger for receiving thecombustion products, including the condensed vapors, from the heatexchanger and separating the condensed vapors from the remaining gasproducts, and control means are associated with the combustion chamberfor terminating the oxygen supply and supplying an inert gas to thecombustion chamber upon completion of the burning of each sample so asto sweep any residual combustion products out of the chamber and onthrough the heat exchanger into the separating means. Thus, in theillustrative system, a resilient connector 50 is provided at the lowerend of the heat exchanger 41 for connecting the outlet of the fluidpassageway member 42 to a conventional sample or counting vial 51. Thevial 51 is supported on a platform 52 which is biased upwardly againstthe connector 50 by means of a biasing spring 53 to provide a gas-tightseal around the upper periphery of the vial. As the combustion productsare discharged from the lower end of the heat exchanger 41, they flowdownwardly into the sample vial 51 so that the liquids are retained inthe vial by gravity, while the gases continue on through a dischargepassageway 54 formed in the resilient connector 50.

When the combustion of a given sample has been completed, the valve 23is closed to terminate the oxygen supply to the combustion chamber, anda valve 60 is opened to supply an inert gas such as nitrogen to thecombustion chamber via the same flow meter 24 and passageways 25, 26previously used to supply the oxygen. This inert gas, which is suppliedunder a slight pressure, sweeps upwardly through the combustion chamber18 so as to purge the chamber of any remaining combustion products, andcontinues on through the chamber exit 27, the transfer tube 34, and theheat exchanger 41. Consequently, it can be seen that the entire systemfrom the combustion chamber 18 to the sample vial 51 is immediatelypurged of all gaseous combustion products following each samplecombustion, and the purging gas also tends to sweep any remaining liquidcondensate out of the heat exchanger. Moreover, since the inert purginggas is discharged from the heat exchanger 41 into the headspace of thesample vial 51 which is used as a part of the liquid-gas separatingmeans, it may also be used to purge oxygen from the vial headspace toavoid the quenching effect of such oxygen during analysis of theresultant sample for radioactivity. Thus, when the sample vial 51 isdisconnected from the resilient connector 50 to place a sealing cap onthe vial, the throat of the vial may be maintained directly under thenitrogen discharge from the connector 50 by simply tilting the viallaterally, so that the nitrogen purges the headspace of the vial bydisplacing any oxygen remaining therein to the atmosphere. As will beapparent to those familiar with this art, this is an important featurebecause oxygen is a severe quenching agent, i.e., it distorts theradioactivity measurements made by liquid scintillation countingtechniques unless certain steps are taken to compensate for the effectof the quenching agent. Although several means of compensating for suchquenching effects are known, they complicate the radioactivity measuringprocedure.

After the purging of the combustion chamber and the heat exchanger, theinert purging gass is preferably turned off by closing the valve 60, andthe inlet of the heat exchanger 41 may be sequentially connected to apair of liquid supply systems generally indicated at 61 and 62. Thefirst supply system 61 includes a supply vessel 63 containing a liquidsolvent of the type conventionally used in the preparation of samples tobe subjected to sub-freezing temperatures, so as to maintain the samplein a liquid state. It will be understood that this first liquid supplysystem 61 is not normally used in the preparation of samples to behandled at above-freezing temperature. Referring now more specificallyto the liquid supply system 61, an inert gas such as nitrogen issupplied to the headspace of the supply vessel 63 under a slightpressure, so as to force the liquid solvent through a valve 64 into ametering dispenser 65 including a movable piston 65a. As long as thevalve 64 remains in the position illustrated in FIG. 1, the piston 65ain the metering dispenser 65 remains in the position illustrated in FIG.1 and no liquid flows out of the dispenser because the output thereof iseffectively closed. However, when the valve 64 is turned 90° to itssecond position, the pressure of the fluid from the supply vessel 63urges the piston 65a to the left as viewed in FIG. 1 until it reaches apreselected stop position, thereby metering a preselected quantity ofliquid through the valve 64 and the T connection 40 into the heatexchanger 41. As the piston 65a moves to the left, the supply of liquidwithin the dispenser 65 is continuously replenished through the righthand end thereof. Thus, when the metered quantity of liquid solvent hasbeen dispensed, the system is ready to dispense the same preselectedquantity of liquid the next time the valve 64 is turned 90°. It will beunderstood that the piston 65a moves alternately to the left and to theright during successive dispensing operations.

The second liquid supply system 62 is used to feed a preselectedquantity of liquid scintillator into the heat exchanger 41 in the samemanner described above for the solvent supply system 61. Thus, theliquid scintillator is fed from a supply vessel 66 through a four-wayvalve 67 into a metering dispenser 68, and is dispensed alternately fromopposite ends of the dispenser in response to successive 90° turns ofthe valve 67. From the valve 67, the liquid flows into the T connection40 and then downwardly through the heat exchanger 41 into the vial 51.

In order to insure that all the liquid supplied to the T connection 40from the liquid supply systems 61, 62 flows downwardly through the heatexchanger 41, a restriction (not shown) may be formed in the transferline 34 to prevent liquid from backing up into the line 34 from the Tconnection 40. As the liquids from the systems 61, 62 flow downwardlythrough the heat exchanger 41, they are discharged through the connector50 into the sample vial 51, where they are retained along with thecondensed vapors collected previously.

It will be appreciated that the connection of the two liquid supplysystems to the heat exchanger inlet not only provides a convenient meansof supplying these liquids to the sample vial connected to the outlet ofthe heat exchanger, but also insures that substantially all thecondensed vapors are recovered from the walls of the heat exchanger tube42. In this connection, one of the important advantages of theillustrative system is that the radioactive tracer never passes throughany valves or other devices having movable parts, thereby facilitatingrecovery of the tracer and elimination of equipment memory. Moreover,due to the small volume of the heat exchanger, any fluid containedtherein changes at a relatively high rate when fluid is flowingtherethrough. To insure that all the liquids fed into the heat exchanger41 are discharged therefrom, it is preferred to resume the nitrogen flowthrough the heat exchanger, via the combustion chamber, for a shortinterval of about five seconds, for example, after the liquid flow fromthe two systems 61, 62 has been terminated. (This nitrogen flow can alsobe used to purge the headspace of the vial 51 as it is removed from theconnector 50, prior to placement of the cap thereon, in the mannerdescribed previously). With this system, it has been found thatessentially 100% of the radioactive isotope tracer present in thestarting material can be recovered in the sample vial 51, when theisotope is in the form of a condensable vapor.

In accordance with a further important aspect of this invention forrecovering tracers by reaction with a trapping agent, the gases whichare separated from the condensed vapors are passed into a reactioncolumn including means for receiving a liquid trapping agent andreacting the gases with the trapping agent as the gases flow through thecolumn. The reaction column is also provided with means for reversingthe direction of the gas flow through the column for discharging thetrapping agent and the reaction product from the column, and a samplevial is connected to the reaction column for receiving the trappingagent and reaction product from the column in response to the reversalof the gas flow. Thus, in the illustrative system, the gases dischargedfrom the first sample vial 51 through the discharge passageway 54 in theresilient connector 50 are passed through a valve 70 which, when in theposition shown in FIG. 1, conducts the gases through a connector 71 intoa second sample vial 72. From the sample vial 72, the gases enter thelower end of a depending stem 73 of a reaction column 74 comprising aseries of smoothly contoured reaction chambers 74a interconnected bysmoothly contoured necked down portions 74b with the interconnectingwalls of the chambers 74a and the necked down portions 74b forming asmooth curvilinear configuration. When the reaction column 74 is used,i.e., when a radioactive isotope tracer is to be recovered by reactionwith a trapping agent, a valve 75 is turned 90° from the position shownin FIG. 1 so as to feed a preselected amount of liquid trapping agentinto an inlet stem in the middle of the reaction column 74 just afterthe oxygen supply to the combustion chamber 18 is turned on. Thus, gasis already flowing upwardly through the reaction column 74 when theliquid trapping agent first enters the column. With the particularconfiguration of reaction column provided by this invention, it has beenfound that the liquid trapping agent becomes uniformly distributedthroughout the various reaction chambers 74a, and such distribution ismaintained, as long as gas flows continuously up through the column 74.That is, the upward gas flow through the reaction column causes theliquid trapping agent to become distributed along the walls of thebulbous or enlarged reaction chambers 74a while preventing the trappingagent from flowing down through the elongated depending stem 73 at thebottom of the reaction column, so that no liquid trapping agent entersthe vial 72.

In order to feed a preselected amount of trapping agent to the reactioncolumn, the liquid is supplied by means of a metering device 77 having amovable piston therein with a predetermined stop position. Thus, thepiston 77a is stopped at the same position during each feedingoperation, so that the same amount of liquid trapping agent will besupplied for each sample. As long as the valve 75 is in the positionillustrated in FIG. 1, the piston 77a in the metering device 77 remainsin the position shown in FIG. 1 and no liquid flows into the inlet stem76. When the valve is turned 90° clockwise, the output of the device 77is connected to the inlet stem 76, and subsequent advancement of theplunger 77a dispenses a preselected quantity of trapping agent into thereaction column 74; although a manually actuated plunger 77a is shown inthe drawings, it will be understood that the dispensing of the liquidtrapping agent could be made automatically responsive to the turning ofthe valve 75. After the metered amount of trapping agent has been fedinto the reaction column, the valve 75 is returned to its normalposition as illustrated in FIG. 1, and the metering device 77 isautomatically refilled with liquid trapping agent from a supply bottle78, the liquid being fed from the bottle 78 into the metering device 77by means of pressurized nitrogen in the headspace of the supply bottle78.

As the gases containing the radioactive isotope tracer, such as ¹⁴ CO₂for example, are passed upwardly through the reaction column, theradioactive compound is reacted with the trapping agent, such asethanolamine for example, to form a reaction product which is heldwithin the reaction chambers 74a along with the liquid trapping agent.The amount of reaction product contained in the series of reactionchambers 74a varies along the length of the reaction column, but it hasbeen found that the reaction effected by the particular reaction columnconfiguration provided by this invention traps over 99% of the isotopetracer. The unreacted gases are exhausted from the upper end of thereaction column through a connector member 79 and vented to theatmosphere through a valve 80.

To control the reaction temperature within the column 74, a heattransfer fluid is passed through an annular jacket surrounding thecolumn 74. In this connection, it has been found that the reactioncolumn provided by this invention provides effective heat transfer witha high degree of efficiency when used to carry out gas-liquid reactions.It is believed that the interaction of the upwardly flowing gas with theliquid that is held within the enlarged reaction chambers 74a brings allportions of the liquid into intimate contact with the column walls,thereby effecting efficient heat transfer between the liquid and thecolumn walls.

After the sample combustion has been completed, the flow of nitrogen gasis continued for a suitable purging period, e.g. 30 seconds, and thevalve 70 is then turned 90° so as to conduct the purging nitrogen gasinto the upper end of the reaction column 74, thereby effecting areversal of the direction of gas flow through the column. As the gasflows downwardly through the reaction column 74, it sweeps the liquidscontained therein, including the reaction product formed by reaction ofthe liquid trapping agent with the gas compound containing the isotopetracer, into the sample vial 72. The gases are discharged from the vial72 upwardly through the connector 71 and vented to the atmospherethrough the valve 70 via passageway 70a therein.

The valve 70 is then returned to its original position to resume the gasflow upwardly through the reaction column, and the connector 79 at theupper end of the reaction column 74 may be sequentially connected to apair of liquid supply systems generally indicated at 81 and 82. Thefirst supply system 81 includes a supply vessel 83 containing a liquidsolvent to be used to dissolve the reaction product formed by reactionof the isotope compound with the trapping agent; the solvent may alsoserve to maintain the resultant sample in a liquid condition where it isto be handled at sub-freezing temperatures, as described previously inconnection with the liquid supply system 61. An inert gas such asnitrogen is supplied to the headspace of the supply vessel 83 under aslight pressure so as to force the liquid solvent through a valve 84into a metering dispenser 85 including a movable piston 85a. Asdescribed previously in connection with the liquid dispensers 65 and 68,the piston 85a moves back and forth within the dispenser 85 in responseto successive 90° turns of the valve 84, so as to feed a preselectedquantity of liquid solvent through the valve 84 into the connector 79each time the valve 84 is turned 90°. This liquid flows downwardly intothe reaction column 74 and is distributed therethrough in the samemanner described previously for the liquid trapping agent suppliedthrough the inlet stem 76. It has been found that the combination of theupward gas flow and the liquid input at the top of the column, providesa scrubbing action on the inside walls of the reaction column so thatsubstantially all the reaction produce contained therein is recovered inthe sample vial 72. In fact, it has been found that the recoveryeffected by this reaction column is so efficient that it hassubstantially no memory whatever, and over 99% of the isotope tracer isrecovered in the vial 72.

After the first liquid has been dispensed into the top of the reactioncolumn, the nitrogen flow is continued upwardly through the column for aperiod of about 15 to 45 seconds, depending upon the concentration ofCO₂ relative to the trapping agent. The valve 70 is then again turned90° to reverse the gas flow through the reaction column, therebysweeping the liquid solvent downwardly through the reaction column intothe sample vial 72. The valve 70 is then again returned to its originalposition so that the inert purging gas once again flows upwardly throughthe reaction column, and the liquid scintillator is metered into theupper end of the reaction column from the second liquid supply system82. More particularly, liquid scintillator is fed from a supply bottle86 through a four-way valve 87 into a metering dispenser 88. When thevalve 87 is turned 90° from the position illustrated in FIG. 1, with thedispenser piston 88a in the position shown, a preselected quantity ofliquid scintillator is forced out of the dispenser by the pressure ofthe nitrogen in the headspace of the bottle 86, thereby advancing thepiston 88a to the left to force liquid through the valve 87 into theconnector 79 at the top of the reaction column 74. Due to the upward gasflow through the reaction column, this liquid again provides a scrubbingaction on the walls of the reaction column 74. After the liquid has beendispersed into the column, the upward nitrogen flow is continued forabout 5 to 10 seconds, at which time the gas flow is again reversed inthe column 74, by turning the valve 70, to discharge the liquidscintillator into the vial 72. In addition to providing a convenientmeans of admitting the liquid scintillator into the vial 72, the liquidsupply systems associated with the reaction column 74 provide a rapidand efficient means of achieving recoveries in excess of 99% withattendant low memories of 1/1000 or less. Moreover, it will beappreciated that the isotope tracer is passed through only a singlevalve 70, and then only while it is in the gas form, thereby furtherfacilitating complete recovery of the radioactive tracer.

In one example of the invention, ten one-gram samples oftritium-labelled samples were combusted in sequence in the sameequipment, with a blank sample, i.e., a sample containing no radioactivetracer, being combusted after each labelled sample. The combustion ofeach sample was initiated by the electrical igniter, heated to atemperature of about 1500° C., and the oxygen flow rate was set at abouttwo liters per minute. The pressure inside the combustion chamber duringcombustion was less than 0.1 atmosphere above atmospheric pressure. Thewalls of the combustion chamber were pre-heated and thermostaticallymaintained at approximately 170° C. which was sufficient to prevent anynoticeable condensation of the combustion products on the inside wallsof the combustion chamber. During combustion, the combustion productswere continuously exhausted through the upper end of the combustionchamber into a heat exchanger, comprising a straight tube of stainlesssteel having an inside diameter of 0.080 inch, a wall thickness of 0.020inch, and a length of 10.00 inches. The walls of the tube weremaintained at a temperature of about 0° C. From the heat exchanger,condensed vapors including condensed ³ H₂ O dripped into the countingvial connected to the lower end of the heat exchanger, while theremaining gases passed on through the vial and were vented to theatmosphere.

The combustion of each sample was completed in about 45 seconds, afterwhich the oxygen was turned off and the nitrogen supply to thecombustion chamber was turned on so that nitrogen was fed into thecombustion chamber at a rate of seven liters per minute for about fiveto ten seconds. The nitrogen was then shut off and a selected quantityof dioxane (liquid scintillator) was fed from the netering dispenserinto the inlet of the heat exchanger. The metering dispenser was presetto feed ten milliliters of the liquid scintillator into the heatexchanger over a period of about five seconds, after which the liquidsupply line to the inlet of the heat exchanger was closed, and thenitrogen feed to the combustion vessel was resumed for an additionalfive seconds at a rate of about four liters per minute. During thisfinal nitrogen feed, the counting vial was removed from the resilientconnector at the outlet of the heat exchanger and tilted with the openmouth of the vial positioned below the passageway from the heatexchanger outlet so that the nitrogen supplied to the counting vialduring this interval purged the vial of oxygen. The vial cap was thenquickly threaded onto the vial to seal the sample contained therein in anitrogen atmosphere, and the sample was analyzed for radioactivity.

The radioactivity level of the tracer in the starting material placed inthe combustion chamber was 100,000 disintegrations per minute (dpm).When the sample collected in the counting vial was analyzed forradioactivity, a count of 42,000 counts per minute (cpm) was measured.The counting efficiency of the analytical method was determined to be42% so that the measured count of 42,000 cpm indicated that there was noloss whatever, i.e., there was 100% recovery of the radioactivematerial. To check the accuracy of the radioactivity measurement madefor the recovered material, the same amount and type of radioactiveisotope tracer that was injected into the original starting material wasplaced in a second counting vial and analyzed for radioactivity in thesame equipment used to analyze the recovered sample. The count measuredfor this second counting vial was identical to the measurement for thefirst sample, i.e., the count was 42,000 cpm in each case, therebyconfirming that the recovery was in fact 100%. Over the series of tensamples, the standard deviation of recovery was determined to be 0.7%,which is about the same degree of variability accounted for bystatistical variations in the samples plus the accuracy of theanalytical instrument without automatic standardization. Based on acomparison of the counts of the radioactive samples and the alternateblank samples, a memory of 1/10,000 or less was obtained consistentlythroughout the entire series of samples. The 42% counting efficiencycompares with maximum efficiencies of 25% to 36% obtainable bycomparable methods used previously, the improvement being due in largemeasure to the fact that there was little or no oxygen present in thesample so that quenching effects were minimized or perhaps eveneliminated. In addition to the increase in efficiency, there was acorresponding reduction in background, so that the resulting figure ofmerit (efficiency squared divided by background) was significantlyincreased. For example, with the 42% efficiency, the background was 27so that the figure of merit was 650, which compares with a figure ofmerit of 370 obtainable by the conventional previous methods. The totaltime required to prepare the above samples was such that about 30 to 40samples could be prepared per hour.

In another example of the invention, 300 milligrams of double-labelled(³ H and ¹⁴ C) material was placed in the combustion chamber and burnedin the same manner described above in the previous example. The onlydifferences in the combustion step of this example were that the oxygenflow rate was initially set at about 0.1 liter per minute, andimmediately after the oxygen was turned on 2.7 milliliters ofethanolamine (trapping agent) were manually injected into the reactioncolumn. After the ethanolamine was injected into the reaction column,the oxygen feed rate was gradually increased to one liter per minute,and at the same time the pressure in the gas feed line to the reactioncolumn was increased by turning the valve in the atmosphere vent linetoward the closed position until the pressure reached 0.3 atmosphereabove atmospheric pressure. At this point, the sample was ignited in thecombustion chamber in the same manner described in the example above. Awhite flame was initially produced due to the burning of hydrogen (whichburns more rapidly than the carbon and produces a white flame). As thiswhite flame began to diminish, the oxygen flow rate was graduallydecreased to about 0.3 liters per minute to complete the combustion ofthe carbon. The exhaust gases from the combustion chamber were initiallyrich in water due to the combustion of the hydrogen, and subsequentlybecome richer in carbon dioxide due to the combustion of the carbon. Thewater was condensed and collected in the first counting vial, while theCO₂ gas was passed on to the reaction column containing ethanolamine asa trapping agent; as the CO₂ passed upwardly through the reactioncolumn, it reacted with the ethanolamine to form a carbamate reactionproduct. After the combustion was completed, the oxygen was turned offand the nitrogen turned on at a flow rate of 0.3 liters per minute andmaintained for about 15 seconds to purge the system of gaseouscombustion products. With the nitrogen flow continuing, the valveconnected between the first vial and the reaction column was turned toits second position so that the nitrogen flow was fed into the top ofthe reaction column rather than the bottom, thereby reversing thenitrogen flow through the column to sweep the liquid reaction product,as well as any unreacted trapping agent, downwardly through the reactioncolumn and into the second counting vial. The excess gases weredischarged through the connector 71 and the valve 70. After the reactionproduct was collected in the second counting vial, the valve 70 wasreturned to its original position, with the nitrogen flow rate beingmaintained at about 0.3 liter per minute. The nitrogen was then turnedoff, and the liquid scintillator was fed into the inlet of the heatexchanger in the same manner described above. The nitrogen feed was thenturned on again to purge the headspace of the first counting vial in thesame manner described above, after which the vial was then removed andreplaced with a new vial which simply served as a conduit for thenitrogen gas to be flowed through the system for the balance of thepreparation procedure. At this point, the four-way valve associated withthe metering device for the liquid solvent was turned to connect theoutput of the metering device to the top of the reaction column so thatthe pressure in the headspace of the solvent supply bottle forced apreselected quantity of liquid solvent into the reaction column. As thisliquid passed downwardly through the column, the upwardly passingnitrogen gas coacted with the downwardly flowing liquid to create aturbulent condition within the reaction column. The nitrogen flow wasmaintained for 15 seconds and then again switched to the top of thereaction column so as to sweep the liquid downwardly through the columnand into the counting vial. At this point, the valve 70 was returned toits original position so that the nitrogen flow once again entered thebottom of the column and flowed upwardly therethrough. At the same time,the four-way valve associated with the liquid scintillator supply systemwas turned to connect the output of the metering device to the top ofthe reaction column so as to feed a preselected metered amount of liquidscintillator into the top of the reaction column in the same mannerdescribed previously for the liquid solvent, except that the nitrogenflow was maintained for only 5 seconds, after which the valve 70 wasagain switched to its second position to conduct the nitrogen into thetop of the column and sweep the liquid down into the counting vial. Thenitrogen flow rate was then increased to a level of four liters perminute, and the second counting vial was removed from its stopper andtilted thereunder to purge the vial headspace in the same mannerdescribed previously for the first counting vial. At this point, thenitrogen flow was shut off and the sample preparation procedure wascomplete. The amount of liquid solvent fed into the reaction vessel waseight milliliters, and the amount of liquid scintillator was the same.The total sample preparation time for this double-labelled sample wassuch that 10 to 15 samples could be prepared per hour. The countingefficiency was 70%, the recovery was in excess of 99%, the standarddeviation of recovery was 0.9%, and the memory was a maximum of 1/1000.The background was 37, so that the figure of merit was 133.

It will be understood from the foregoing description that theillustrative sample preparation system may be used to prepare samplesfrom starting materials labelled with only a single tracer to berecovered either as a condensed vapor or by reaction with a trappingagent, or from double-labelled samples containing tracers to berecovered by both means. In the event that the material is labelled withonly a single tracer to be recovered as a condensed vapor, the gasesdischarged from the first sample vial 51 are, of course, simply passedon through the balance of the system and vented to the atmosphere. Inthe case of a sample labelled with only a single tracer to be recoveredby reaction with a trapping agent, it is not necessary to supply aliquid scintillator to the heat exchanger 41, although it may be desiredto feed some other liquid through the heat exchanger in order to removethe condensed vapors therefrom between successive combustions.Similarly, there is no need to feed any liquids whatever into thereaction column 74 when the sample is labelled with only a single tracerto be recovered as a condensed vapor, since the gas is discharged fromthe vial 51 will normally be vented to the atmosphere via valve 70during the preparation of such samples. If it is desired to prepare onlytritium-labelled samples, for example, that portion of the systemdownstream of the vial 51 may even be eliminated.

It will also be appreciated that any of the manual operations requiredin the illustrative system may be readily converted to automaticoperation. For example, the opening and closing of the oxygen andnitrogen valves 23 and 60, respectively, may be controlled by timingmechanisms according to a predetermined time schedule for particulartypes of samples. Similarly, the valves 64, 67, 75, 84, and 87associated with the various liquid supply systems, as well as the valve70, could be controlled by timing mechanisms according to predeterminedtime schedules.

As can be seen from the foregoing detailed description, this inventionprovides an improved sample preparation method and apparatus whichreduce the sample preparation time far below the preparation timesrequired by the methods and apparatus previously known for thepreparation of such samples, with corresponding increases in the samplepreparation rate. Consequently, a technician using this system canprepare a much greater number of samples in any given work period,thereby improving the efficiency and reducing the cost of suchpreparation procedures. This invention significantly increases theefficiency of the isotope recovery from the starting material,permitting recoveries of essentially 100% of the isotope present in thestarting material. As a result, the memory of the sample preparationequipment is virtually eliminated, so that the reliability of theresultant samples and the data derived therefrom are greatly improved.The invention also permits the preparation of samples which containlittle or no oxygen, thereby minimizing quenching effects. The improvedheat exchanger used to recover the condensable vapors provides anextremely high heat transfer with only a small volume and surface areaand in a very short time period, and the improved reaction columnachieves a high reaction rate between the gas and liquid for therecovery of isotopes to be reacted in gas form with a liquid trappingagent.

I claim as my invention:
 1. Apparatus for preparing isotope-containingsamples for use in studies utilizing radioactive isotopes, saidapparatus comprising the combination of(a) a combustion chamber forcombusting a sample material containing the isotope tritium to producecombustion products containing tritiated water vapor, (b) means forcontinuously exhausting the tritiated water vapor from said combustionchamber during the combustion of said material, (c) a heat exchanger forcontinuously cooling the exhausted combustion products to convert thetritiated water vapor to a liquid during the combustion of saidmaterial, (d) a sample collection vessel and means for continuouslyremoving said isotope-containing liquid from said heat exchanger duringthe combustion of said material and transferring said liquid to saidcollection vessel to provide a liquid sample containing the recoveredisotopes for use in studies utilizing radioactive isotopes, (e) andmeans for purging said combustion chamber and heat exchanger between thecombustion of successive isotope-containing samples.
 2. Apparatus forpreparing samples as set forth in claim 1 which includes means forcontinuing said exhausting, cooling and removing steps after completionof combustion of the sample material until substantially all the ³ H₂ Ois recovered.
 3. Apparatus for preparing samples as set forth in claim 1which includes means for flushing said combustion chamber with a fluidsubsequent to the combustion step so as to sweep any residual combustionproducts out of said chamber and on through said heat exchanger toachieve substantially complete recovery of the ³ H₂ O while purging thesystem prior to combustion of the next sample.
 4. Apparatus forpreparing samples as set forth in claim 1 which includes a sample vialattached to the exit of the heat exchanger for collecting the liquidremoved from said heat exchanger, and means for directing inert gas fromsaid heat exchanger into said vial so that any oxygen contained in thevial head space may be displaced to the atmosphere when the vial isremoved from the heat exchanger.
 5. Apparatus for preparing samples asset forth in claim 1 which includes means for preheating the walls ofthe combustion chamber prior to the combustion steps so as to maintainsaid walls above the condensation temperature of the vapors contained inthe combustion products.
 6. Apparatus for preparing samples as set forthin claim 1 which includes means for maintaining said combustion productsabove the condensation temperature of the vapors contained therein untilsaid combustion products are cooled in said heat exchanger.
 7. Apparatusfor preparing samples as set forth in claim 1 which includes means forsupplying a liquid to the inlet end of said heat exchanger subsequent tothe combustion step so that said liquid flows through the heat exchangerto recover any condensed vapors remaining therein.
 8. Apparatus forpreparing samples as set forth in claim 7 which includes means forsupplying a liquid scintillator and a liquid solvent sequentially to theinlet end of said heat exchanger.
 9. Apparatus for preparingisotope-containing samples for use in studies utilizing radioactiveisotopes, said apparatus comprising the combination of(a) a combustionchamber for combusting a sample material containing a radioactiveisotope to produce combustion products containing the radioactiveisotope in the form of a gaseous product containing tritiated watervapor, said combustion chamber having sidewalls extending upwardly andinwardly above said sample of material so as to approximate the shape ofthe flame of a burning sample to minimize the oxygen-containingatmosphere around the flame, (b) means for continuously exhausting saidisotope-containing combustion products from said combustion chamberduring the combustion of said material, (c) a heat exchanger forcontinuously cooling the exhausted combustion products to condense anytritiated water vapor and other condensable vapors contained thereinduring the combustion of said material, (d) a sample collection vesseland means for continuously separating the condensed isotope-containingvapors from the remaining gaseous combustion products during thecombustion of said material and transferring the condensed vapors tosaid collection vessel to provide a liquid sample containing therecovered isotopes for use in studies utilizing radioactive isotopes,(e) and means for purging said combustion chamber and heat exchangerbetween the combustion of successive isotope-containing samples. 10.Apparatus for preparing samples as set forth in claim 9 which includesmeans for continuing said exhausting, cooling, and separating stepsafter completion of the combustion of the sample material untilsubstantially all the combustion products are removed from saidcombustion chamber.
 11. Apparatus for preparing samples as set forth inclaim 9 which includes means for flushing said combustion chamber with afluid subsequent to the combustion step so as to sweep any residualcombustion products out of said chamber and on through said heatexchanger to achieve substantially complete recovery of the isotopeswhile purging the system prior to combustion of the next sample. 12.Apparatus as set forth in claim 9 wherein said heat exchanger comprisesa tube made of thermally conductive material and forming an elongatedfluid passageway designed to provide laminar flow of gases and vaporspassing therethrough in the absence of condensation, the cross sectionof said passageway being sufficiently small in at least one directiontransverse to the fluid flow to provide a pulsating pressure at theinlet end of said passageway in the presence of condensation therein,the walls of said passageway being maintained at a temperature at leastas low as the condensation temperature of the vapors passingtherethrough.
 13. Apparatus for preparing isotope-containing samples foruse in studies utilizing radioactive isotopes, said apparatus comprisingthe combination of(a) a combustion chamber for combusting a sample ofmaterial containing a radioactive isotope to produce combustion productscontaining the radioactive isotope in the form of a gaseous productcontaining tritiated water vapor, (b) means for continuously exhaustingsaid isotope-containing products from said combustion chamber during thecombustion of said material, (c) a heat exchanger for continuouslycooling the exhausted combustion products to condense any tritiatedwater vapor and other condensable vapors contained therein during thecombustion of said material, said heat exchanger comprising a tube madeof thermally conductive material and forming an elongated fluidpassageway designed to provide laminar flow of gases and vapors passingtherethrough in the absence of condensation, the cross section of saidpassageway being sufficiently small in at least one direction transverseto the fluid flow to provide a pulsating fluid pressure at the inlet endof said passageway in the presence of condensation therein, the walls ofsaid passageway being maintained at a temperature at least as low as thecondensation temperature of the vapors passing therethrough, (d) asample collection vessel and means for continuously separating thecondensed isotope-containing vapors from the remaining combustionproducts during the combustion of said material and transferring thecondensed vapors to said collection vessel to provide a liquid samplecontaining the recovered isotope for use in studies utilizingradioactive isotopes, (e) and means for purging said combustion chamberand heat exchanger between the combustion of successiveisotope-containing samples.
 14. Apparatus for preparing samples as setforth in claim 13 which includes means for continuing said exhausting,cooling, and separating steps after completion of the burning of thesample material until substantially all the combustion products areremoved from said combustion chamber.
 15. Apparatus for preparingsamples as set forth in claim 13 which includes means for flushing saidcombustion chamber and said heat exchanger with a fluid subsequent tothe combustion step to achieve substantially complete recovery of theisotope while purging the system prior to combustion of the next sample.16. Apparatus for preparing isotope-containing samples for use instudies utilizing radioactive isotopes, said apparatus comprising thecombination of(a) a combustion chamber for combusting a sample ofmaterial containing ³ H and ¹⁴ C to produce combustion productscontaining ³ H₂ O and ¹⁴ CO₂ in gaseous form, (b) means for continuouslyexhausting the ³ H₂ O and ¹⁴ CO₂ gas from said combustion chamber duringthe combustion of said material, (c) a heat exchanger for continuouslycooling the exhausted combustion products to convert the ³ H₂ O gas to aliquid during the combustion of said material, (d) a sample collectionvessel and means for continuously removing said liquid from said heatexchanger during the combustion of said material and transferring saidliquid to said collection vessel to provide a liquid sample containingthe recovered ³ H₂ O for use in studies utilizing radioactive isotopes,(e) means for maintaining the ¹⁴ CO₂ in gaseous form during recovery ofthe ³ H₂ O in liquid form, (f) a trapping chamber for continuouslycontacting the ¹⁴ CO₂ with a trapping agent to convert the ¹⁴ CO₂ to a¹⁴ C-containing liquid during the combustion of said material, (g) asecond sample collection vessel and means for removing the resulting ¹⁴C-containing liquid from said trapping chamber to recover the ¹⁴ C andtransferring said liquid to a sample collection vessel to provide aliquid sample containing the recovered ¹⁴ CO₂ for use in studiesutilizing radioactive isotopes, (h) and means for purging saidcombustion chamber, heat exchanger and trapping chamber between thecombustion of successive isotope-containing samples.
 17. Apparatus forpreparing samples as set forth in claim 16 which includes means forflushing said combustion chamber with a fluid subsequent to thecombustion step so as to sweep any residual combustion products out ofsaid chamber and on through said heat exchanger and trapping column toachieve substantially complete recovery of the ³ H₂ O and substantiallycomplete trapping of the ¹⁴ CO₂ while purging the system prior tocombustion of the next sample, and means for flushing said trappingchamber with a fluid subsequent to the removal of the ¹⁴ C-containingliquid therefrom to achieve substantially complete recovery of the ¹⁴ Cwhile purging the trapping chamber prior to combustion of the nextsample.
 18. Apparatus as set forth in claim 16 wherein said trappingchamber comprises a series of smoothly contoured bulbous chambers eachadjacent pair of which are interconnected by a smoothly contoured neckeddown portion with the interconnecting walls of said bulbous chambers andsaid necked down portions forming a smooth curvilinear configurationwhereby said liquid is distributed along the length of said column insaid bulbous chambers while gas is flowing therethrough and iseffectively mixed with the gas flowing therethrough to effect a reactiontherebetween, whereby fractional interaction between said liquid andsaid gas stream is effected along the length of said trapping chamber.19. Apparatus for preparing isotope-containing samples for use instudies utilizing radioactive isotopes, said apparatus comprising thecombination of(a) a combustion chamber for combusting a sample ofmaterial containing ¹⁴ C to produce combustion products containing ¹⁴CO₂ in gaseous form, (b) means for continuously exhausting said ¹⁴ CO₂-containing combustion products from said combustion chamber during thecombustion of said material, (c) a trapping column for continuouslycontacting the exhausted combustion products with a trapping agent toconvert the ¹⁴ CO₂ to a ¹⁴ C-containing liquid during the combustion ofsaid material, said trapping column comprising a series of smoothlycontoured bulbous chambers each adjacent pair of which areinterconnected by a smoothly contoured necked down portion with theinterconnecting walls of said chambers and said necked down portionsforming a smooth curvilinear configuration whereby said liquid isdistributed along the length of said column in said chambers while gasis flowing therethrough and is effectively mixed with the gas flowingtherethrough to effect a reaction therebetween, whereby fractionalinteraction between said liquid and said gas stream is effected alongthe length of said column, (d) and a sample collection vessel and meansfor removing said liquid from said trapping column to recover the ¹⁴ Cand transferring said liquid to said collection vessel to provide aliquid sample containing a recovered isotope for use in studiesutilizing radioactive isotopes, (e) and means for purging saidcombustion chamber and trapping column between the combustion ofsuccessive isotope-containing samples.
 20. Apparatus for preparingsamples as set forth in claim 19 which includes means for continuingsaid exhausting and contacting steps after completion of the combustionof the sample material until substantially all the ¹⁴ CO₂ is recoveredfrom the combustion chamber.
 21. Apparatus for preparing samples as setforth in claim 19 which includes means for flushing said combustionchamber with a fluid subsequent to the combustion step so as to sweepany residual combustion products out of said chamber and on through saidtrapping column to achieve substantially complete trapping of the ¹⁴ CO₂while purging the system prior to combustion of the next sample, andmeans for flushing said trapping column with a fluid subsequent to theremoval of the ¹⁴ C-containing liquid therefrom to achieve substantiallycomplete recovery of the ¹⁴ C while purging the trapping column prior tocombustion of the next sample.
 22. Apparatus for preparing samples asset forth in claim 19 which includes means for supplying oxygen to saidcombustion chamber at a controlled rate during the combustion of saidmaterial.